Meeting Abstract

43.2  Wednesday, Jan. 5  Morphological Integration of the Turtle Shell MELSTROM, K. M.*; ANGIELCZYK, K. D.; University of Michigan; Field Museum kmelstro@umich.edu

The turtle shell is one of the most distinctive evolutionary novelties among tetrapods, but there is significant variation in shell morphology and function. In particular, previous research has demonstrated that the presence of a hinge in the plastron (ventral shell) has a strong effect on plastron shape. Here we investigated whether plastral kinesis also influences patterns of morphological integration. We digitized two-dimensional landmarks on the plastra of over 1800 turtles belonging to kinetic and akinetic species in Emydidae, Geoemydidae, and Kinosternidae. Analysis of this data set confirmed the strong effect of kinesis on plastron shape. We then used Klingenberg’s method to determine whether covariation within a series of hypothesized modules was stronger than in alternative subdivisions of landmarks. The number of hypothesized modules varied from two to four, with particular attention paid to modules that are separated by the plastral hinge. The best supported modularity hypotheses for most of the hinged turtles consisted of two modules divided by the hinge, despite the fact that the hinged species belong to several clades that independently evolved this character. Several modularity patterns were apparent in the akinetic turtles, but none corresponded exactly to the patterns in the kinetic species. The evolution of a plastral hinge is accompanied by sterotyped changes in plastron shape and morphology related to the development of a flexible joint between the anterior and posterior portions of the plastron and the replacement of the bony bridge between the plastron and carapace with a ligamentous connection. Our results suggest that this sterotypy extends to patterns of integration, suggesting that there may be developmental or other constraints on how a kinetic plastron can evolve.